1. Field of the Invention
This invention relates to precision laser welding, marking or cutting. Pulses of laser energy are triggered on the change in position of the galvo scanner used to direct the laser energy to the workpiece or change in position of the galvo and the workpiece.
2. Description of Related Art
Apparatus for accurately positioning a laser beam on a workpiece surface for various purposes are known in the art. For example, Overbeck U.S. Pat. No. 4,532,402 entitled “Method and Apparatus for Positioning a Focused Beam on an Integrated Circuit” discloses an X-Y positioning table for moving the workpiece into the field of view of a galvanometer beam positioning system. The galvanometer (typically referred to as a “galvo”) comprises two mirrors mounted for rotation. The mirrors are separately controlled by drive assembles including scanning motors for directing a laser beam over the workpiece. Control of the mirrors is coordinated with the movement of the X-Y positioning table supporting the workpiece. Interferometer position feedback from the X-Y positioning table is passed to the galvanometer drive assembles making it unnecessary to move the X-Y positioning table over the workpiece and await settling of the table at the stopped position.
Precision laser welding wherein the overlapped weld beads are applied along a weld seam is used in various applications, for example, in production of implantable medical devices. Where a hermetically sealed weld seam is required, overlapping weld beads are laid down along the weld seam as disclosed, for example, in Papenfuss et al. U.S. Application Publication No. 2006/0144827 entitled “Method and Apparatus for Laser Welding Incorporating Galvanometer Delivery.” This published patent application teaches that the laser is pulsed at a fixed frequency, say 10 Hertz, while the galvanometer directs the beam along the weld path. This represents the current state of the art known to applicant. The laser pulses are timed by a constant frequency clock that is asynchronous to the motion of the scanner mirrors and/or an X-Y positioning table supporting the workpiece. The frequency of the laser pulses is based on the laser spot size, desired spot overlap, and the programmed full speed in one direction of the scan over the workpiece. This approach results in varying spot overlap at the beginning and end of programmed motion because the speed varies during the acceleration and deceleration of the galvanometer mirrors. One attempt to overcome the problem is to change the frequency of the laser pulses along the beam path. This reduces but does not eliminate the overlap variation. This procedure can introduce variability in pulse placement as the ideal point at which to modify the pulse frequency does not coincide with the change in speed of the scan over the workpiece.